This invention relates to a method for the lubrication of a parison for use in a blow molding process. Utilization of the present invention permits blow molding of containers having elliptical horizontal cross sections that have a high aspect ratio.
The production of containers through the use of a blow molding process is widely practiced in the container manufacturing industry. The first step in this process consists of extruding or, preferably, injecting a molten substance into a mold cavity. Upon hardening, a thick walled hollow object commonly known as a preform or a parison is produced. This parison is then brought to a temperature conducive to blow molding and placed in a blow mold. A high pressure gas or mixture of gasses such as air is then injected into the interior of the parison, causing the parison to rapidly expand until it contacts the walls of the blow mold cavity. The final form of the container is produced by the walls of the blow mold cavity cooling the substance and preventing further expansion of the substance.
Many substances can be used in a blow molding process, but glass and thermoplastics are most commonly used in the container manufacturing industry. High density polyethylene, polypropylene, and polyvinyl chloride based plastics are all suitable for use in a blow molding process. For some thermoplastics, such polyethylene terepthalate, an additional step of biaxially orienting the thermoplastic by stretching the parison can provide superior clarity, drop resistance, and tensile strength to the finished product.
However, blow molding is not an ideal method for producing certain types of finished articles that have shapes whose cross section is not substantially circular. Containers having elliptical cross sections of high aspect ratio are difficult to produce using this method because the parison walls freeze against the mold walls upon contact, stopping any flow of material from the frozen areas of the parison to those areas of the parison that are still expanding due to air pressure. As a result, the walls of the container are abnormally thick in those areas that first contact the mold near the minor axis of the mold, and abnormally thin in those areas near the major axis of the mold.
A further problem is the migration of the crystallized plastic sprue point when thermoplastics such as polyethylene terepthalate are used. When the parison is injection molded, the plastic nearest the injection sprue is abnormally hot compared to the rest of the plastic. The heat in the sprue area causes polyethylene terepthalate to crystallize into a more brittle form. This crystallized plastic, sometimes known as a sprue artifact, constitutes an unavoidable weak point in the finished product. However, the possibility of failure at the point of crystallization represented by the sprue artifact can be minimized by appropriate design of the finished product. If the sprue artifact is centered at the bottom of the finished product, and protected from direct contact with other objects by raised ridges or feet, the possibility of failure due to impact is signnificantly reduced.
When products having a substantially circular cross section are blow molded, the sprue artifact remains generally centered in a protected position because the blown parison nearly simultaneously contacts the walls of the blow mold cavity. However, if the mold has a high aspect ratio elliptical horizontal cross section, the sprue artifact can migrate to areas outside the protective structures due to non-uniform thinning caused by freezing of the plastic to the mold walls. If the sprue artifact migrates outside of the protected area, the finished product is useless because of the high risk of stress induced container failure at the sprue artifact point.
Several methods have unsuccessfully been attempted to remedy the problem of wall thinning and migration of the sprue artifact. Varying blow pressures, changing mold temperatures and changing mold profiles have failed to alleviate the problems associated with blow molding containers having a high aspect ratio elliptical, horizontal cross sections. More substantial changes, such as forming a blow mold from nickel impregnated with a slippery fluorocarbon compound such as Teflon have also failed to prevent wall thinning due to premature freezing contact with the mold walls. Other alternatives such as designing preforms with elliptical shapes could work in certain situations, but the cost of retooling to handle and properly orient variant shapes can be prohibitive, especially when low volume or marginally profitable production runs are contemplated.